Run tests on travis against an up-to-date nightly

Running "nightly" on trusty (the current default on travis) is not
nightly any more, but 3.7.0a4+.  See
https://docs.travis-ci.com/user/languages/python/#development-releases-support

Author: michael-k

.travis.yml

@@ -4,13 +4,15 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
-  - "nightly"
   # - "pypy" - won't work as smmap doesn't work (see gitdb/.travis.yml for details)
 matrix:
   include:
     - python: 3.7
       dist: xenial
       sudo: required
+    - python: "nightly"
+      dist: xenial
+      sudo: required
   allow_failures:
     - python: "nightly"
 git: